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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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 - Copyright (C) 2004-2007  Internet Systems Consortium, Inc. ("ISC")
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 -
 - Permission to use, copy, modify, and distribute this software for any
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<!-- File: $Id: Bv9ARM-book.xml,v 1.326 2007/05/28 00:09:07 marka Exp $ -->
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<book xmlns:xi="http://www.w3.org/2001/XInclude">
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  <title>BIND 9 Administrator Reference Manual</title>

  <bookinfo>
    <copyright>
      <year>2004</year>
      <year>2005</year>
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      <year>2006</year>
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      <year>2007</year>
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      <holder>Internet Systems Consortium, Inc. ("ISC")</holder>
    </copyright>
    <copyright>
      <year>2000</year>
      <year>2001</year>
      <year>2002</year>
      <year>2003</year>
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      <holder>Internet Software Consortium.</holder>
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    </copyright>
  </bookinfo>

  <chapter id="Bv9ARM.ch01">
    <title>Introduction</title>
    <para>
      The Internet Domain Name System (<acronym>DNS</acronym>)
      consists of the syntax
      to specify the names of entities in the Internet in a hierarchical
      manner, the rules used for delegating authority over names, and the
      system implementation that actually maps names to Internet
      addresses.  <acronym>DNS</acronym> data is maintained in a
      group of distributed
      hierarchical databases.
    </para>

    <sect1>
      <title>Scope of Document</title>

      <para>
        The Berkeley Internet Name Domain
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        (<acronym>BIND</acronym>) implements a
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        domain name server for a number of operating systems. This
        document provides basic information about the installation and
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        care of the Internet Systems Consortium (<acronym>ISC</acronym>)
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        <acronym>BIND</acronym> version 9 software package for
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        system administrators.
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      </para>
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      <para>
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        This version of the manual corresponds to BIND version 9.4.
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      </para>
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    </sect1>
    <sect1>
      <title>Organization of This Document</title>
      <para>
        In this document, <emphasis>Section 1</emphasis> introduces
        the basic <acronym>DNS</acronym> and <acronym>BIND</acronym> concepts. <emphasis>Section 2</emphasis>
        describes resource requirements for running <acronym>BIND</acronym> in various
        environments. Information in <emphasis>Section 3</emphasis> is
        <emphasis>task-oriented</emphasis> in its presentation and is
        organized functionally, to aid in the process of installing the
        <acronym>BIND</acronym> 9 software. The task-oriented
        section is followed by
        <emphasis>Section 4</emphasis>, which contains more advanced
        concepts that the system administrator may need for implementing
        certain options. <emphasis>Section 5</emphasis>
        describes the <acronym>BIND</acronym> 9 lightweight
        resolver.  The contents of <emphasis>Section 6</emphasis> are
        organized as in a reference manual to aid in the ongoing
        maintenance of the software. <emphasis>Section 7</emphasis> addresses
        security considerations, and
        <emphasis>Section 8</emphasis> contains troubleshooting help. The
        main body of the document is followed by several
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        <emphasis>appendices</emphasis> which contain useful reference
        information, such as a <emphasis>bibliography</emphasis> and
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        historic information related to <acronym>BIND</acronym>
        and the Domain Name
        System.
      </para>
    </sect1>
    <sect1>
      <title>Conventions Used in This Document</title>

      <para>
        In this document, we use the following general typographic
        conventions:
      </para>

      <informaltable>
        <tgroup cols="2">
          <colspec colname="1" colnum="1" colwidth="3.000in"/>
          <colspec colname="2" colnum="2" colwidth="2.625in"/>
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          <tbody>
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            <row>
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              <entry colname="1">
                <para>
                  <emphasis>To describe:</emphasis>
                </para>
              </entry>
              <entry colname="2">
                <para>
                  <emphasis>We use the style:</emphasis>
                </para>
              </entry>
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            </row>
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            <row>
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              <entry colname="1">
                <para>
                  a pathname, filename, URL, hostname,
                  mailing list name, or new term or concept
                </para>
              </entry>
              <entry colname="2">
                <para>
                  <filename>Fixed width</filename>
                </para>
              </entry>
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            </row>
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            <row>
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              <entry colname="1">
                <para>
                  literal user
                  input
                </para>
              </entry>
              <entry colname="2">
                <para>
                  <userinput>Fixed Width Bold</userinput>
                </para>
              </entry>
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            </row>
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            <row>
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              <entry colname="1">
                <para>
                  program output
                </para>
              </entry>
              <entry colname="2">
                <para>
                  <computeroutput>Fixed Width</computeroutput>
                </para>
              </entry>
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            </row>
          </tbody>
        </tgroup>
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      </informaltable>

      <para>
        The following conventions are used in descriptions of the
        <acronym>BIND</acronym> configuration file:<informaltable colsep="0" frame="all" rowsep="0">
                  <tgroup cols="2" colsep="0" rowsep="0" tgroupstyle="2Level-table">
                      <colspec colname="1" colnum="1" colsep="0" colwidth="3.000in"/>
            <colspec colname="2" colnum="2" colsep="0" colwidth="2.625in"/>
            <tbody>
              <row rowsep="0">
                <entry colname="1" colsep="1" rowsep="1">
                  <para>
                    <emphasis>To describe:</emphasis>
                  </para>
                </entry>
                <entry colname="2" rowsep="1">
                  <para>
                    <emphasis>We use the style:</emphasis>
                  </para>
                </entry>
              </row>
              <row rowsep="0">
                <entry colname="1" colsep="1" rowsep="1">
                  <para>
                    keywords
                  </para>
                </entry>
                <entry colname="2" rowsep="1">
                  <para>
                    <literal>Fixed Width</literal>
                  </para>
                </entry>
              </row>
              <row rowsep="0">
                <entry colname="1" colsep="1" rowsep="1">
                  <para>
                    variables
                  </para>
                </entry>
                <entry colname="2" rowsep="1">
                  <para>
                    <varname>Fixed Width</varname>
                  </para>
                </entry>
              </row>
              <row rowsep="0">
                <entry colname="1" colsep="1">
                  <para>
                    Optional input
                  </para>
                </entry>
                <entry colname="2">
                  <para>
                    <optional>Text is enclosed in square brackets</optional>
                  </para>
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
    </sect1>
    <sect1>
      <title>The Domain Name System (<acronym>DNS</acronym>)</title>
      <para>
        The purpose of this document is to explain the installation
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        and upkeep of the <acronym>BIND</acronym> (Berkeley Internet
	Name Domain) software package, and we
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        begin by reviewing the fundamentals of the Domain Name System
        (<acronym>DNS</acronym>) as they relate to <acronym>BIND</acronym>.
      </para>

      <sect2>
        <title>DNS Fundamentals</title>

        <para>
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          The Domain Name System (DNS) is a hierarchical, distributed
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          database.  It stores information for mapping Internet host names to
          IP
          addresses and vice versa, mail routing information, and other data
          used by Internet applications.
        </para>

        <para>
          Clients look up information in the DNS by calling a
          <emphasis>resolver</emphasis> library, which sends queries to one or
          more <emphasis>name servers</emphasis> and interprets the responses.
          The <acronym>BIND</acronym> 9 software distribution
          contains a
          name server, <command>named</command>, and two resolver
          libraries, <command>liblwres</command> and <command>libbind</command>.
        </para>

        </sect2><sect2>
        <title>Domains and Domain Names</title>

        <para>
          The data stored in the DNS is identified by <emphasis>domain names</emphasis> that are organized as a tree according to
          organizational or administrative boundaries. Each node of the tree,
          called a <emphasis>domain</emphasis>, is given a label. The domain
          name of the
          node is the concatenation of all the labels on the path from the
          node to the <emphasis>root</emphasis> node.  This is represented
          in written form as a string of labels listed from right to left and
          separated by dots. A label need only be unique within its parent
          domain.
        </para>

        <para>
          For example, a domain name for a host at the
          company <emphasis>Example, Inc.</emphasis> could be
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          <literal>ourhost.example.com</literal>,
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          where <literal>com</literal> is the
          top level domain to which
          <literal>ourhost.example.com</literal> belongs,
          <literal>example</literal> is
          a subdomain of <literal>com</literal>, and
          <literal>ourhost</literal> is the
          name of the host.
        </para>

        <para>
          For administrative purposes, the name space is partitioned into
          areas called <emphasis>zones</emphasis>, each starting at a node and
          extending down to the leaf nodes or to nodes where other zones
          start.
          The data for each zone is stored in a <emphasis>name server</emphasis>, which answers queries about the zone using the
          <emphasis>DNS protocol</emphasis>.
        </para>

        <para>
          The data associated with each domain name is stored in the
          form of <emphasis>resource records</emphasis> (<acronym>RR</acronym>s).
          Some of the supported resource record types are described in
          <xref linkend="types_of_resource_records_and_when_to_use_them"/>.
        </para>

        <para>
          For more detailed information about the design of the DNS and
          the DNS protocol, please refer to the standards documents listed in
          <xref linkend="rfcs"/>.
        </para>
      </sect2>

      <sect2>
        <title>Zones</title>
        <para>
          To properly operate a name server, it is important to understand
          the difference between a <emphasis>zone</emphasis>
          and a <emphasis>domain</emphasis>.
        </para>

        <para>
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          As stated previously, a zone is a point of delegation in
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          the <acronym>DNS</acronym> tree. A zone consists of
          those contiguous parts of the domain
          tree for which a name server has complete information and over which
          it has authority. It contains all domain names from a certain point
          downward in the domain tree except those which are delegated to
          other zones. A delegation point is marked by one or more
          <emphasis>NS records</emphasis> in the
          parent zone, which should be matched by equivalent NS records at
          the root of the delegated zone.
        </para>

        <para>
          For instance, consider the <literal>example.com</literal>
          domain which includes names
          such as <literal>host.aaa.example.com</literal> and
          <literal>host.bbb.example.com</literal> even though
          the <literal>example.com</literal> zone includes
          only delegations for the <literal>aaa.example.com</literal> and
          <literal>bbb.example.com</literal> zones.  A zone can
          map
          exactly to a single domain, but could also include only part of a
          domain, the rest of which could be delegated to other
          name servers. Every name in the <acronym>DNS</acronym>
          tree is a
          <emphasis>domain</emphasis>, even if it is
          <emphasis>terminal</emphasis>, that is, has no
          <emphasis>subdomains</emphasis>.  Every subdomain is a domain and
          every domain except the root is also a subdomain. The terminology is
          not intuitive and we suggest that you read RFCs 1033, 1034 and 1035
          to
          gain a complete understanding of this difficult and subtle
          topic.
        </para>

        <para>
          Though <acronym>BIND</acronym> is called a "domain name
          server",
          it deals primarily in terms of zones. The master and slave
          declarations in the <filename>named.conf</filename> file
          specify
          zones, not domains. When you ask some other site if it is willing to
          be a slave server for your <emphasis>domain</emphasis>, you are
          actually asking for slave service for some collection of zones.
        </para>
      </sect2>

      <sect2>
        <title>Authoritative Name Servers</title>

        <para>
          Each zone is served by at least
          one <emphasis>authoritative name server</emphasis>,
          which contains the complete data for the zone.
          To make the DNS tolerant of server and network failures,
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          most zones have two or more authoritative servers, on
          different networks.
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        </para>

        <para>
          Responses from authoritative servers have the "authoritative
          answer" (AA) bit set in the response packets.  This makes them
          easy to identify when debugging DNS configurations using tools like
          <command>dig</command> (<xref linkend="diagnostic_tools"/>).
        </para>

        <sect3>
          <title>The Primary Master</title>

          <para>
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            The authoritative server where the master copy of the zone
            data is maintained is called the
	    <emphasis>primary master</emphasis> server, or simply the
            <emphasis>primary</emphasis>.  Typically it loads the zone
            contents from some local file edited by humans or perhaps
            generated mechanically from some other local file which is
            edited by humans.  This file is called the
	    <emphasis>zone file</emphasis> or
	    <emphasis>master file</emphasis>.
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          </para>
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	  <para>
	    In some cases, however, the master file may not be edited
	    by humans at all, but may instead be the result of
	    <emphasis>dynamic update</emphasis> operations.
	  </para>
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        </sect3>

        <sect3>
          <title>Slave Servers</title>
          <para>
            The other authoritative servers, the <emphasis>slave</emphasis>
            servers (also known as <emphasis>secondary</emphasis> servers)
            load
            the zone contents from another server using a replication process
            known as a <emphasis>zone transfer</emphasis>.  Typically the data
            are
            transferred directly from the primary master, but it is also
            possible
            to transfer it from another slave.  In other words, a slave server
            may itself act as a master to a subordinate slave server.
          </para>
        </sect3>

        <sect3>
          <title>Stealth Servers</title>

          <para>
            Usually all of the zone's authoritative servers are listed in
            NS records in the parent zone.  These NS records constitute
            a <emphasis>delegation</emphasis> of the zone from the parent.
            The authoritative servers are also listed in the zone file itself,
            at the <emphasis>top level</emphasis> or <emphasis>apex</emphasis>
            of the zone.  You can list servers in the zone's top-level NS
            records that are not in the parent's NS delegation, but you cannot
            list servers in the parent's delegation that are not present at
            the zone's top level.
          </para>

          <para>
            A <emphasis>stealth server</emphasis> is a server that is
            authoritative for a zone but is not listed in that zone's NS
            records.  Stealth servers can be used for keeping a local copy of
            a
            zone to speed up access to the zone's records or to make sure that
            the
            zone is available even if all the "official" servers for the zone
            are
            inaccessible.
          </para>

          <para>
            A configuration where the primary master server itself is a
            stealth server is often referred to as a "hidden primary"
            configuration.  One use for this configuration is when the primary
            master
            is behind a firewall and therefore unable to communicate directly
            with the outside world.
          </para>

        </sect3>

      </sect2>
      <sect2>

        <title>Caching Name Servers</title>

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	<!--
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	  - Terminology here is inconsistent.  Probably ought to
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	  - convert to using "recursive name server" everywhere
	  - with just a note about "caching" terminology.
	  -->

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        <para>
          The resolver libraries provided by most operating systems are
          <emphasis>stub resolvers</emphasis>, meaning that they are not
          capable of
          performing the full DNS resolution process by themselves by talking
          directly to the authoritative servers.  Instead, they rely on a
          local
          name server to perform the resolution on their behalf.  Such a
          server
          is called a <emphasis>recursive</emphasis> name server; it performs
          <emphasis>recursive lookups</emphasis> for local clients.
        </para>

        <para>
          To improve performance, recursive servers cache the results of
          the lookups they perform.  Since the processes of recursion and
          caching are intimately connected, the terms
          <emphasis>recursive server</emphasis> and
          <emphasis>caching server</emphasis> are often used synonymously.
        </para>

        <para>
          The length of time for which a record may be retained in
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          the cache of a caching name server is controlled by the
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          Time To Live (TTL) field associated with each resource record.
        </para>

        <sect3>
          <title>Forwarding</title>

          <para>
            Even a caching name server does not necessarily perform
            the complete recursive lookup itself.  Instead, it can
            <emphasis>forward</emphasis> some or all of the queries
            that it cannot satisfy from its cache to another caching name
            server,
            commonly referred to as a <emphasis>forwarder</emphasis>.
          </para>

          <para>
            There may be one or more forwarders,
            and they are queried in turn until the list is exhausted or an
            answer
            is found. Forwarders are typically used when you do not
            wish all the servers at a given site to interact directly with the
            rest of
            the Internet servers. A typical scenario would involve a number
            of internal <acronym>DNS</acronym> servers and an
            Internet firewall. Servers unable
            to pass packets through the firewall would forward to the server
            that can do it, and that server would query the Internet <acronym>DNS</acronym> servers
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            on the internal server's behalf.
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          </para>
        </sect3>

      </sect2>

      <sect2>
        <title>Name Servers in Multiple Roles</title>

        <para>
          The <acronym>BIND</acronym> name server can
          simultaneously act as
          a master for some zones, a slave for other zones, and as a caching
          (recursive) server for a set of local clients.
        </para>

        <para>
          However, since the functions of authoritative name service
          and caching/recursive name service are logically separate, it is
          often advantageous to run them on separate server machines.

          A server that only provides authoritative name service
          (an <emphasis>authoritative-only</emphasis> server) can run with
          recursion disabled, improving reliability and security.

          A server that is not authoritative for any zones and only provides
          recursive service to local
          clients (a <emphasis>caching-only</emphasis> server)
          does not need to be reachable from the Internet at large and can
          be placed inside a firewall.
        </para>

      </sect2>
    </sect1>

  </chapter>

  <chapter id="Bv9ARM.ch02">
    <title><acronym>BIND</acronym> Resource Requirements</title>

    <sect1>
      <title>Hardware requirements</title>

      <para>
        <acronym>DNS</acronym> hardware requirements have
        traditionally been quite modest.
        For many installations, servers that have been pensioned off from
        active duty have performed admirably as <acronym>DNS</acronym> servers.
      </para>
      <para>
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        The DNSSEC features of <acronym>BIND</acronym> 9
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        may prove to be quite
        CPU intensive however, so organizations that make heavy use of these
        features may wish to consider larger systems for these applications.
        <acronym>BIND</acronym> 9 is fully multithreaded, allowing
        full utilization of
        multiprocessor systems for installations that need it.
      </para>
    </sect1>
    <sect1>
      <title>CPU Requirements</title>
      <para>
        CPU requirements for <acronym>BIND</acronym> 9 range from
        i486-class machines
        for serving of static zones without caching, to enterprise-class
        machines if you intend to process many dynamic updates and DNSSEC
        signed zones, serving many thousands of queries per second.
      </para>
    </sect1>

    <sect1>
      <title>Memory Requirements</title>
      <para>
        The memory of the server has to be large enough to fit the
        cache and zones loaded off disk.  The <command>max-cache-size</command>
        option can be used to limit the amount of memory used by the cache,
        at the expense of reducing cache hit rates and causing more <acronym>DNS</acronym>
        traffic.
        Additionally, if additional section caching
        (<xref linkend="acache"/>) is enabled,
        the <command>max-acache-size</command> can be used to
        limit the amount
        of memory used by the mechanism.
        It is still good practice to have enough memory to load
        all zone and cache data into memory &mdash; unfortunately, the best
        way
        to determine this for a given installation is to watch the name server
        in operation. After a few weeks the server process should reach
        a relatively stable size where entries are expiring from the cache as
        fast as they are being inserted.
      </para>
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      <!--
        - Add something here about leaving overhead for attacks?
	- How much overhead?  Percentage?
        -->
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    </sect1>

    <sect1>
      <title>Name Server Intensive Environment Issues</title>
      <para>
        For name server intensive environments, there are two alternative
        configurations that may be used. The first is where clients and
        any second-level internal name servers query a main name server, which
        has enough memory to build a large cache. This approach minimizes
        the bandwidth used by external name lookups. The second alternative
        is to set up second-level internal name servers to make queries
        independently.
        In this configuration, none of the individual machines needs to
        have as much memory or CPU power as in the first alternative, but
        this has the disadvantage of making many more external queries,
        as none of the name servers share their cached data.
      </para>
    </sect1>

    <sect1>
      <title>Supported Operating Systems</title>
      <para>
        ISC <acronym>BIND</acronym> 9 compiles and runs on a large
        number
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        of Unix-like operating system and on NT-derived versions of
        Microsoft Windows such as Windows 2000 and Windows XP.  For an
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        up-to-date
        list of supported systems, see the README file in the top level
        directory
        of the BIND 9 source distribution.
      </para>
    </sect1>
  </chapter>

  <chapter id="Bv9ARM.ch03">
    <title>Name Server Configuration</title>
    <para>
      In this section we provide some suggested configurations along
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      with guidelines for their use.  We suggest reasonable values for
      certain option settings.
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    </para>

    <sect1 id="sample_configuration">
      <title>Sample Configurations</title>
      <sect2>
        <title>A Caching-only Name Server</title>
        <para>
          The following sample configuration is appropriate for a caching-only
          name server for use by clients internal to a corporation.  All
          queries
          from outside clients are refused using the <command>allow-query</command>
          option.  Alternatively, the same effect could be achieved using
          suitable
          firewall rules.
        </para>
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<programlisting>
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// Two corporate subnets we wish to allow queries from.
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acl corpnets { 192.168.4.0/24; 192.168.7.0/24; };
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options {
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     directory "/etc/namedb";           // Working directory
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     allow-query { corpnets; };
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};
// Provide a reverse mapping for the loopback address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
     type master;
     file "localhost.rev";
     notify no;
};
</programlisting>
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      </sect2>

      <sect2>
        <title>An Authoritative-only Name Server</title>
        <para>
          This sample configuration is for an authoritative-only server
          that is the master server for "<filename>example.com</filename>"
          and a slave for the subdomain "<filename>eng.example.com</filename>".
        </para>
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<programlisting>
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options {
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     directory "/etc/namedb";           // Working directory
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     allow-query-cache { none; };       // Do not allow access to cache
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     allow-query { any; };              // This is the default
     recursion no;                      // Do not provide recursive service
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};

// Provide a reverse mapping for the loopback address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
     type master;
     file "localhost.rev";
     notify no;
};
// We are the master server for example.com
zone "example.com" {
     type master;
     file "example.com.db";
     // IP addresses of slave servers allowed to transfer example.com
     allow-transfer {
          192.168.4.14;
          192.168.5.53;
     };
};
// We are a slave server for eng.example.com
zone "eng.example.com" {
     type slave;
     file "eng.example.com.bk";
     // IP address of eng.example.com master server
     masters { 192.168.4.12; };
};
</programlisting>
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      </sect2>
    </sect1>

    <sect1>
      <title>Load Balancing</title>
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      <!--
        - Add explanation of why load balancing is fragile at best
	- and completely pointless in the general case.
        -->
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      <para>
        A primitive form of load balancing can be achieved in
        the <acronym>DNS</acronym> by using multiple A records for
        one name.
      </para>

      <para>
        For example, if you have three WWW servers with network addresses
        of 10.0.0.1, 10.0.0.2 and 10.0.0.3, a set of records such as the
        following means that clients will connect to each machine one third
        of the time:
      </para>

      <informaltable colsep="0" rowsep="0">
        <tgroup cols="5" colsep="0" rowsep="0" tgroupstyle="2Level-table">
          <colspec colname="1" colnum="1" colsep="0" colwidth="0.875in"/>
          <colspec colname="2" colnum="2" colsep="0" colwidth="0.500in"/>
          <colspec colname="3" colnum="3" colsep="0" colwidth="0.750in"/>
          <colspec colname="4" colnum="4" colsep="0" colwidth="0.750in"/>
          <colspec colname="5" colnum="5" colsep="0" colwidth="2.028in"/>
          <tbody>
            <row rowsep="0">
              <entry colname="1">
                <para>
                  Name
                </para>
              </entry>
              <entry colname="2">
                <para>
                  TTL
                </para>
              </entry>
              <entry colname="3">
                <para>
                  CLASS
                </para>
              </entry>
              <entry colname="4">
                <para>
                  TYPE
                </para>
              </entry>
              <entry colname="5">
                <para>
                  Resource Record (RR) Data
                </para>
              </entry>
            </row>
            <row rowsep="0">
              <entry colname="1">
                <para>
                  <literal>www</literal>
                </para>
              </entry>
              <entry colname="2">
                <para>
                  <literal>600</literal>
                </para>
              </entry>
              <entry colname="3">
                <para>
                  <literal>IN</literal>
                </para>
              </entry>
              <entry colname="4">
                <para>
                  <literal>A</literal>
                </para>
              </entry>
              <entry colname="5">
                <para>
                  <literal>10.0.0.1</literal>
                </para>
              </entry>
            </row>
            <row rowsep="0">
              <entry colname="1">
                <para/>
              </entry>
              <entry colname="2">
                <para>
                  <literal>600</literal>
                </para>
              </entry>
              <entry colname="3">
                <para>
                  <literal>IN</literal>
                </para>
              </entry>
              <entry colname="4">
                <para>
                  <literal>A</literal>
                </para>
              </entry>
              <entry colname="5">
                <para>
                  <literal>10.0.0.2</literal>
                </para>
              </entry>
            </row>
            <row rowsep="0">
              <entry colname="1">
                <para/>
              </entry>
              <entry colname="2">
                <para>
                  <literal>600</literal>
                </para>
              </entry>
              <entry colname="3">
                <para>
                  <literal>IN</literal>
                </para>
              </entry>
              <entry colname="4">
                <para>
                  <literal>A</literal>
                </para>
              </entry>
              <entry colname="5">
                <para>
                  <literal>10.0.0.3</literal>
                </para>
              </entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>
      <para>
        When a resolver queries for these records, <acronym>BIND</acronym> will rotate
        them and respond to the query with the records in a different
        order.  In the example above, clients will randomly receive
        records in the order 1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients
        will use the first record returned and discard the rest.
      </para>
      <para>
        For more detail on ordering responses, check the
        <command>rrset-order</command> substatement in the
        <command>options</command> statement, see
        <xref endterm="rrset_ordering_title" linkend="rrset_ordering"/>.
      </para>

    </sect1>

    <sect1>
      <title>Name Server Operations</title>

      <sect2>
        <title>Tools for Use With the Name Server Daemon</title>
        <para>
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          This section describes several indispensable diagnostic,
          administrative and monitoring tools available to the system
          administrator for controlling and debugging the name server
          daemon.
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        </para>
        <sect3 id="diagnostic_tools">
          <title>Diagnostic Tools</title>
          <para>
            The <command>dig</command>, <command>host</command>, and
            <command>nslookup</command> programs are all command
            line tools
            for manually querying name servers.  They differ in style and
            output format.
          </para>

          <variablelist>
            <varlistentry>
              <term id="dig"><command>dig</command></term>
              <listitem>
                <para>
                  The domain information groper (<command>dig</command>)
                  is the most versatile and complete of these lookup tools.
                  It has two modes: simple interactive
                  mode for a single query, and batch mode which executes a
                  query for
                  each in a list of several query lines. All query options are
                  accessible
                  from the command line.
                </para>
                <cmdsynopsis label="Usage">
                  <command>dig</command>
                  <arg>@<replaceable>server</replaceable></arg>
                  <arg choice="plain"><replaceable>domain</replaceable></arg>
                  <arg><replaceable>query-type</replaceable></arg>
                  <arg><replaceable>query-class</replaceable></arg>
                  <arg>+<replaceable>query-option</replaceable></arg>
                  <arg>-<replaceable>dig-option</replaceable></arg>
                  <arg>%<replaceable>comment</replaceable></arg>
                </cmdsynopsis>
                <para>
                  The usual simple use of dig will take the form
                </para>
                <simpara>
                  <command>dig @server domain query-type query-class</command>
                </simpara>
                <para>
                  For more information and a list of available commands and
                  options, see the <command>dig</command> man
                  page.
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><command>host</command></term>
              <listitem>
                <para>
                  The <command>host</command> utility emphasizes
                  simplicity
                  and ease of use.  By default, it converts
                  between host names and Internet addresses, but its
                  functionality
                  can be extended with the use of options.
                </para>
                <cmdsynopsis label="Usage">
                  <command>host</command>
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                  <arg>-aCdlnrsTwv</arg>
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                  <arg>-c <replaceable>class</replaceable></arg>
                  <arg>-N <replaceable>ndots</replaceable></arg>
                  <arg>-t <replaceable>type</replaceable></arg>
                  <arg>-W <replaceable>timeout</replaceable></arg>
                  <arg>-R <replaceable>retries</replaceable></arg>
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                  <arg>-m <replaceable>flag</replaceable></arg>
		  <arg>-4</arg>
		  <arg>-6</arg>
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                  <arg choice="plain"><replaceable>hostname</replaceable></arg>
                  <arg><replaceable>server</replaceable></arg>
                </cmdsynopsis>
                <para>
                  For more information and a list of available commands and
                  options, see the <command>host</command> man
                  page.
                </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term><command>nslookup</command></term>
              <listitem>
                <para><command>nslookup</command>
		  has two modes: interactive and
                  non-interactive. Interactive mode allows the user to
                  query name servers for information about various
                  hosts and domains or to print a list of hosts in a
                  domain. Non-interactive mode is used to print just
                  the name and requested information for a host or
                  domain.
                </para>
                <cmdsynopsis label="Usage">
                  <command>nslookup</command>
                  <arg rep="repeat">-option</arg>
                  <group>
                    <arg><replaceable>host-to-find</replaceable></arg>
                    <arg>- <arg>server</arg></arg>
                  </group>
                </cmdsynopsis>
                <para>
                  Interactive mode is entered when no arguments are given (the
                  default name server will be used) or when the first argument
                  is a
                  hyphen (`-') and the second argument is the host name or
                  Internet address
                  of a name server.
                </para>
                <para>
                  Non-interactive mode is used when the name or Internet
                  address
                  of the host to be looked up is given as the first argument.
                  The
                  optional second argument specifies the host name or address
                  of a name server.
                </para>
                <para>
                  Due to its arcane user interface and frequently inconsistent
                  behavior, we do not recommend the use of <command>nslookup</command>.
                  Use <command>dig</command> instead.
                </para>
              </listitem>

            </varlistentry>
          </variablelist>
        </sect3>

        <sect3 id="admin_tools">
          <title>Administrative Tools</title>
          <para>
            Administrative tools play an integral part in the management
            of a server.
          </para>
          <variablelist>
            <varlistentry id="named-checkconf" xreflabel="Named Configuration Checking application">

              <term><command>named-checkconf</command></term>
              <listitem>
                <para>
                  The <command>named-checkconf</command> program
                  checks the syntax of a <filename>named.conf</filename> file.
                </para>
                <cmdsynopsis label="Usage">
                  <command>named-checkconf</command>
                  <arg>-jvz</arg>
                  <arg>-t <replaceable>directory</replaceable></arg>
                  <arg><replaceable>filename</replaceable></arg>
                </cmdsynopsis>
              </listitem>
            </varlistentry>
            <varlistentry id="named-checkzone" xreflabel="Zone Checking application">

              <term><command>named-checkzone</command></term>
              <listitem>
                <para>
                  The <command>named-checkzone</command> program
                  checks a master file for
                  syntax and consistency.
                </para>
                <cmdsynopsis label="Usage">
                  <command>named-checkzone</command>
                  <arg>-djqvD</arg>
                  <arg>-c <replaceable>class</replaceable></arg>
                  <arg>-o <replaceable>output</replaceable></arg>
                  <arg>-t <replaceable>directory</replaceable></arg>
                  <arg>-w <replaceable>directory</replaceable></arg>
                  <arg>-k <replaceable>(ignore|warn|fail)</replaceable></arg>
                  <arg>-n <replaceable>(ignore|warn|fail)</replaceable></arg>
                  <arg>-W <replaceable>(ignore|warn)</replaceable></arg>
                  <arg choice="plain"><replaceable>zone</replaceable></arg>
                  <arg><replaceable>filename</replaceable></arg>
                </cmdsynopsis>
              </listitem>
            </varlistentry>
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	    <varlistentry id="named-compilezone" xreflabel="Zone Compilation aplication">
	      <term><command>named-compilezone</command></term>
	      <listitem>
		<para>
		  Similar to <command>named-checkzone,</command> but
		  it always dumps the zone content to a specified file
		  (typically in a different format).
		</para>
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	      </listitem>
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	    </varlistentry>
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            <varlistentry id="rndc" xreflabel="Remote Name Daemon Control application">

              <term><command>rndc</command></term>
              <listitem>
                <para>
                  The remote name daemon control
                  (<command>rndc</command>) program allows the
                  system
                  administrator to control the operation of a name server.
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                  Since <acronym>BIND</acronym> 9.2, <command>rndc</command>
                  supports all the commands of the BIND 8 <command>ndc</command>
                  utility except <command>ndc start</command> and
                  <command>ndc restart</command>, which were also
                  not supported in <command>ndc</command>'s
                  channel mode.
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                  If you run <command>rndc</command> without any
                  options
                  it will display a usage message as follows:
                </para>
                <cmdsynopsis label="Usage">
                  <command>rndc</command>
                  <arg>-c <replaceable>config</replaceable></arg>
                  <arg>-s <replaceable>server</replaceable></arg>
                  <arg>-p <replaceable>port</replaceable></arg>
                  <arg>-y <replaceable>key</replaceable></arg>
                  <arg choice="plain"><replaceable>command</replaceable></arg>
                  <arg rep="repeat"><replaceable>command</replaceable></arg>
                </cmdsynopsis>
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                <para>The <command>command</command>
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		  is one of the following:
                </para>

                <variablelist>

                  <varlistentry>
                    <term><userinput>reload</userinput></term>
                    <listitem>
                      <para>
                        Reload configuration file and zones.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>reload <replaceable>zone</replaceable>
                        <optional><replaceable>class</replaceable>
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           <optional><replaceable>view</replaceable></optional></optional></userinput></term>
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                    <listitem>
                      <para>
                        Reload the given zone.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>refresh <replaceable>zone</replaceable>
                        <optional><replaceable>class</replaceable>
           <optional><replaceable>view</replaceable></optional></optional></userinput></term>
                    <listitem>
                      <para>
                        Schedule zone maintenance for the given zone.
                      </para>
                    </listitem>
                  </varlistentry>
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                  <varlistentry>
                    <term><userinput>retransfer <replaceable>zone</replaceable>

                        <optional><replaceable>class</replaceable>
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           <optional><replaceable>view</replaceable></optional></optional></userinput></term>
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                    <listitem>
                      <para>
                        Retransfer the given zone from the master.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
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                    <term><userinput>freeze
                        <optional><replaceable>zone</replaceable>
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       <optional><replaceable>class</replaceable>
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           <optional><replaceable>view</replaceable></optional></optional></optional></userinput></term>
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                    <listitem>
                      <para>
                        Suspend updates to a dynamic zone.  If no zone is
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                        specified,
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                        then all zones are suspended.  This allows manual
                        edits to be made to a zone normally updated by dynamic
                        update.  It
                        also causes changes in the journal file to be synced
                        into the master
                        and the journal file to be removed.  All dynamic
                        update attempts will
                        be refused while the zone is frozen.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>thaw
                        <optional><replaceable>zone</replaceable>
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       <optional><replaceable>class</replaceable>
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           <optional><replaceable>view</replaceable></optional></optional></optional></userinput></term>
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                    <listitem>
                      <para>
                        Enable updates to a frozen dynamic zone.  If no zone
                        is
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                        specified, then all frozen zones are enabled.  This
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                        causes
                        the server to reload the zone from disk, and
                        re-enables dynamic updates
                        after the load has completed.  After a zone is thawed,
                        dynamic updates
                        will no longer be refused.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>notify <replaceable>zone</replaceable>
                        <optional><replaceable>class</replaceable>
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           <optional><replaceable>view</replaceable></optional></optional></userinput></term>
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                    <listitem>
                      <para>
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                        Resend NOTIFY messages for the zone.
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                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>reconfig</userinput></term>
                    <listitem>
                      <para>
                        Reload the configuration file and load new zones,
                        but do not reload existing zone files even if they
                        have changed.
                        This is faster than a full <command>reload</command> when there
                        is a large number of zones because it avoids the need
                        to examine the
                        modification times of the zones files.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>stats</userinput></term>
                    <listitem>
                      <para>
                        Write server statistics to the statistics file.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>querylog</userinput></term>
                    <listitem>
                      <para>
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                        Toggle query logging. Query logging can also be enabled
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                        by explicitly directing the <command>queries</command>
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                        <command>category</command> to a
		        <command>channel</command> in the
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                        <command>logging</command> section of
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                        <filename>named.conf</filename> or by specifying
			<command>querylog yes;</command> in the
			<command>options</command> section of
			<filename>named.conf</filename>.
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                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>dumpdb
                        <optional>-all|-cache|-zone</optional>
                        <optional><replaceable>view ...</replaceable></optional></userinput></term>
                    <listitem>
                      <para>
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                        Dump the server's caches (default) and/or zones to
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                        the
                        dump file for the specified views.  If no view is
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                        specified, all
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                        views are dumped.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>stop <optional>-p</optional></userinput></term>
                    <listitem>
                      <para>
                        Stop the server, making sure any recent changes
                        made through dynamic update or IXFR are first saved to
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                        the master files of the updated zones.
			If -p is specified named's process id is returned.
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			This allows an external process to determine when named
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			had completed stopping.
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                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>halt <optional>-p</optional></userinput></term>
                    <listitem>
                      <para>
                        Stop the server immediately.  Recent changes
                        made through dynamic update or IXFR are not saved to
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                        the master files, but will be rolled forward from the
			journal files when the server is restarted.
			If -p is specified named's process id is returned.
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			This allows an external process to determine when named
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			had completed halting.
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                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>trace</userinput></term>
                    <listitem>
                      <para>
                        Increment the servers debugging level by one.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>trace <replaceable>level</replaceable></userinput></term>
                    <listitem>
                      <para>
                        Sets the server's debugging level to an explicit
                        value.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>notrace</userinput></term>
                    <listitem>
                      <para>
                        Sets the server's debugging level to 0.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>flush</userinput></term>
                    <listitem>
                      <para>
                        Flushes the server's cache.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>flushname</userinput> <replaceable>name</replaceable></term>
                    <listitem>
                      <para>
                        Flushes the given name from the server's cache.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>status</userinput></term>
                    <listitem>
                      <para>
                        Display status of the server.
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                        Note that the number of zones includes the internal <command>bind/CH</command> zone
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                        and the default <command>./IN</command>
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                        hint zone if there is not an
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                        explicit root zone configured.
                      </para>
                    </listitem>
                  </varlistentry>

                  <varlistentry>
                    <term><userinput>recursing</userinput></term>
                    <listitem>
                      <para>
                        Dump the list of queries named is currently recursing
                        on.
                      </para>
                    </listitem>
                  </varlistentry>

                </variablelist>

                <para>
                  A configuration file is required, since all
                  communication with the server is authenticated with
                  digital signatures that rely on a shared secret, and
                  there is no way to provide that secret other than with a
                  configuration file.  The default location for the
                  <command>rndc</command> configuration file is
                  <filename>/etc/rndc.conf</filename>, but an
                  alternate
                  location can be specified with the <option>-c</option>
                  option.  If the configuration file is not found,
                  <command>rndc</command> will also look in
                  <filename>/etc/rndc.key</filename> (or whatever
                  <varname>sysconfdir</varname> was defined when
                  the <acronym>BIND</acronym> build was
                  configured).
                  The <filename>rndc.key</filename> file is
                  generated by
                  running <command>rndc-confgen -a</command> as
                  described in
                  <xref linkend="controls_statement_definition_and_usage"/>.
                </para>

                <para>
                  The format of the configuration file is similar to
                  that of <filename>named.conf</filename>, but
                  limited to
                  only four statements, the <command>options</command>,
                  <command>key</command>, <command>server</command> and
                  <command>include</command>
                  statements.  These statements are what associate the
                  secret keys to the servers with which they are meant to
                  be shared.  The order of statements is not
                  significant.
                </para>

                <para>
                  The <command>options</command> statement has
                  three clauses:
                  <command>default-server</command>, <command>default-key</command>,
                  and <command>default-port</command>.
                  <command>default-server</command> takes a
                  host name or address argument  and represents the server
                  that will
                  be contacted if no <option>-s</option>
                  option is provided on the command line.
                  <command>default-key</command> takes
                  the name of a key as its argument, as defined by a <command>key</command> statement.
                  <command>default-port</command> specifies the
                  port to which
                  <command>rndc</command> should connect if no
                  port is given on the command line or in a
                  <command>server</command> statement.
                </para>

                <para>
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                  The <command>key</command> statement defines a
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                  key to be used
                  by <command>rndc</command> when authenticating
                  with
                  <command>named</command>.  Its syntax is
                  identical to the
                  <command>key</command> statement in named.conf.
                  The keyword <userinput>key</userinput> is
                  followed by a key name, which must be a valid
                  domain name, though it need not actually be hierarchical;
                  thus,
                  a string like "<userinput>rndc_key</userinput>" is a valid
                  name.
                  The <command>key</command> statement has two
                  clauses:
                  <command>algorithm</command> and <command>secret</command>.
                  While the configuration parser will accept any string as the
                  argument
                  to algorithm, currently only the string "<userinput>hmac-md5</userinput>"
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                  has any meaning.  The secret is a base-64 encoded string
		  as specified in RFC 3548.
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                </para>

                <para>
                  The <command>server</command> statement
                  associates a key
                  defined using the <command>key</command>
                  statement with a server.
                  The keyword <userinput>server</userinput> is followed by a
                  host name or address.  The <command>server</command> statement
                  has two clauses: <command>key</command> and <command>port</command>.
                  The <command>key</command> clause specifies the
                  name of the key
                  to be used when communicating with this server, and the
                  <command>port</command> clause can be used to
                  specify the port <command>rndc</command> should
                  connect
                  to on the server.
                </para>

                <para>
                  A sample minimal configuration file is as follows:
                </para>

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<programlisting>
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key rndc_key {
     algorithm "hmac-md5";
     secret "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
};
options {
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     default-server 127.0.0.1;
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     default-key    rndc_key;
};
</programlisting>
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                <para>
                  This file, if installed as <filename>/etc/rndc.conf</filename>,
                  would allow the command:
                </para>
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                <para>
                  <prompt>$ </prompt><userinput>rndc reload</userinput>
                </para>

                <para>
                  to connect to 127.0.0.1 port 953 and cause the name server
                  to reload, if a name server on the local machine were
                  running with
                  following controls statements:
                </para>
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<programlisting>
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controls {
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        inet 127.0.0.1 allow { localhost; } keys { rndc_key; };
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};
</programlisting>

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                <para>
                  and it had an identical key statement for
                  <literal>rndc_key</literal>.
                </para>

                <para>
                  Running the <command>rndc-confgen</command>
                  program will
                  conveniently create a <filename>rndc.conf</filename>
                  file for you, and also display the
                  corresponding <command>controls</command>
                  statement that you need to
                  add to <filename>named.conf</filename>.
                  Alternatively,
                  you can run <command>rndc-confgen -a</command>
                  to set up
                  a <filename>rndc.key</filename> file and not
                  modify
                  <filename>named.conf</filename> at all.
                </para>

              </listitem>
            </varlistentry>
          </variablelist>

        </sect3>
      </sect2>
      <sect2>

        <title>Signals</title>
        <para>
          Certain UNIX signals cause the name server to take specific
          actions, as described in the following table.  These signals can
          be sent using the <command>kill</command> command.
        </para>
        <informaltable frame="all">
          <tgroup cols="2">
            <colspec colname="1" colnum="1" colsep="0" colwidth="1.125in"/>
            <colspec colname="2" colnum="2" colsep="0" colwidth="4.000in"/>
            <tbody>
              <row rowsep="0">
                <entry colname="1">
                  <para><command>SIGHUP</command></para>
                </entry>
                <entry colname="2">
                  <para>
                    Causes the server to read <filename>named.conf</filename> and
                    reload the database.
                  </para>
                </entry>
              </row>
              <row rowsep="0">
                <entry colname="1">
                  <para><command>SIGTERM</command></para>
                </entry>
                <entry colname="2">
                  <para>
                    Causes the server to clean up and exit.
                  </para>
                </entry>
              </row>
              <row rowsep="0">
                <entry colname="1">
                  <para><command>SIGINT</command></para>
                </entry>
                <entry colname="2">
                  <para>
                    Causes the server to clean up and exit.
                  </para>
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </sect2>
    </sect1>
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  </chapter>

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  <chapter id="Bv9ARM.ch04">
    <title>Advanced DNS Features</title>

    <sect1 id="notify">

      <title>Notify</title>
      <para>
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        <acronym>DNS</acronym> NOTIFY is a mechanism that allows master
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        servers to notify their slave servers of changes to a zone's data. In
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        response to a <command>NOTIFY</command> from a master server, the
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        slave will check to see that its version of the zone is the
        current version and, if not, initiate a zone transfer.
      </para>

      <para>
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        For more information about <acronym>DNS</acronym>
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        <command>NOTIFY</command>, see the description of the
        <command>notify</command> option in <xref linkend="boolean_options"/> and
        the description of the zone option <command>also-notify</command> in
        <xref linkend="zone_transfers"/>.  The <command>NOTIFY</command>
        protocol is specified in RFC 1996.
      </para>

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      <note>
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	As a slave zone can also be a master to other slaves, named,
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        by default, sends <command>NOTIFY</command> messages for every zone
	it loads.  Specifying <command>notify master-only;</command> will
	cause named to only send <command>NOTIFY</command> for master
	zones that it loads.
      </note>

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    </sect1>

    <sect1 id="dynamic_update">
      <title>Dynamic Update</title>

      <para>
        Dynamic Update is a method for adding, replacing or deleting
        records in a master server by sending it a special form of DNS
        messages.  The format and meaning of these messages is specified
        in RFC 2136.
      </para>

      <para>
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	Dynamic update is enabled by including an
	<command>allow-update</command> or <command>update-policy</command>
	clause in the <command>zone</command> statement.  The
	<command>tkey-gssapi-credential</command> and
	<command>tkey-domain</command> clauses in the
	<command>options</command>        statement enable the
	server to negotiate keys that can be matched against those
	in <command>update-policy</command> or
	<command>allow-update</command>.
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      </para>

      <para>
        Updating of secure zones (zones using DNSSEC) follows
        RFC 3007: RRSIG and NSEC records affected by updates are automatically
            regenerated by the server using an online zone key.
        Update authorization is based
        on transaction signatures and an explicit server policy.
      </para>

      <sect2 id="journal">
        <title>The journal file</title>

        <para>
          All changes made to a zone using dynamic update are stored
          in the zone's journal file.  This file is automatically created
          by the server when the first dynamic update takes place.
          The name of the journal file is formed by appending the extension
          <filename>.jnl</filename> to the name of the
          corresponding zone
          file unless specifically overridden.  The journal file is in a
          binary format and should not be edited manually.
        </para>

        <para>
          The server will also occasionally write ("dump")
          the complete contents of the updated zone to its zone file.
          This is not done immediately after
          each dynamic update, because that would be too slow when a large
          zone is updated frequently.  Instead, the dump is delayed by
          up to 15 minutes, allowing additional updates to take place.
        </para>

        <para>
          When a server is restarted after a shutdown or crash, it will replay
              the journal file to incorporate into the zone any updates that
          took
          place after the last zone dump.
        </para>

        <para>
          Changes that result from incoming incremental zone transfers are
          also
          journalled in a similar way.
        </para>

        <para>
          The zone files of dynamic zones cannot normally be edited by
          hand because they are not guaranteed to contain the most recent
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          dynamic changes &mdash; those are only in the journal file.
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          The only way to ensure that the zone file of a dynamic zone
          is up to date is to run <command>rndc stop</command>.
        </para>

        <para>
          If you have to make changes to a dynamic zone
          manually, the following procedure will work: Disable dynamic updates
              to the zone using
          <command>rndc freeze <replaceable>zone</replaceable></command>.
          This will also remove the zone's <filename>.jnl</filename> file
          and update the master file.  Edit the zone file.  Run
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          <command>rndc thaw <replaceable>zone</replaceable></command>
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          to reload the changed zone and re-enable dynamic updates.
        </para>

      </sect2>

    </sect1>

    <sect1 id="incremental_zone_transfers">
      <title>Incremental Zone Transfers (IXFR)</title>

      <para>
        The incremental zone transfer (IXFR) protocol is a way for
        slave servers to transfer only changed data, instead of having to
        transfer the entire zone. The IXFR protocol is specified in RFC
        1995. See <xref linkend="proposed_standards"/>.
      </para>

      <para>
        When acting as a master, <acronym>BIND</acronym> 9
        supports IXFR for those zones
        where the necessary change history information is available. These
        include master zones maintained by dynamic update and slave zones
        whose data was obtained by IXFR.  For manually maintained master
        zones, and for slave zones obtained by performing a full zone
        transfer (AXFR), IXFR is supported only if the option
        <command>ixfr-from-differences</command> is set
        to <userinput>yes</userinput>.
      </para>

      <para>
        When acting as a slave, <acronym>BIND</acronym> 9 will
        attempt to use IXFR unless
        it is explicitly disabled. For more information about disabling
        IXFR, see the description of the <command>request-ixfr</command> clause
        of the <command>server</command> statement.
      </para>
    </sect1>

    <sect1>
      <title>Split DNS</title>
      <para>
        Setting up different views, or visibility, of the DNS space to
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        internal and external resolvers is usually referred to as a
	<emphasis>Split DNS</emphasis> setup. There are several
        reasons an organization would want to set up its DNS this way.
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      </para>
      <para>
        One common reason for setting up a DNS system this way is
        to hide "internal" DNS information from "external" clients on the
        Internet. There is some debate as to whether or not this is actually
        useful.
        Internal DNS information leaks out in many ways (via email headers,
        for example) and most savvy "attackers" can find the information
        they need using other means.
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	However, since listing addresses of internal servers that
        external clients cannot possibly reach can result in
        connection delays and other annoyances, an organization may
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        choose to use a Split DNS to present a consistent view of itself
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        to the outside world.
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      </para>
      <para>
        Another common reason for setting up a Split DNS system is
        to allow internal networks that are behind filters or in RFC 1918
        space (reserved IP space, as documented in RFC 1918) to resolve DNS
        on the Internet. Split DNS can also be used to allow mail from outside
        back in to the internal network.
      </para>
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     <sect2>
      <title>Example split DNS setup</title>
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      <para>
        Let's say a company named <emphasis>Example, Inc.</emphasis>
        (<literal>example.com</literal>)
        has several corporate sites that have an internal network with
        reserved
        Internet Protocol (IP) space and an external demilitarized zone (DMZ),
        or "outside" section of a network, that is available to the public.
      </para>
      <para>
        <emphasis>Example, Inc.</emphasis> wants its internal clients
        to be able to resolve external hostnames and to exchange mail with
        people on the outside. The company also wants its internal resolvers
        to have access to certain internal-only zones that are not available
        at all outside of the internal network.
      </para>
      <para>
        In order to accomplish this, the company will set up two sets
        of name servers. One set will be on the inside network (in the
        reserved
        IP space) and the other set will be on bastion hosts, which are
        "proxy"
        hosts that can talk to both sides of its network, in the DMZ.
      </para>
      <para>
        The internal servers will be configured to forward all queries,
        except queries for <filename>site1.internal</filename>, <filename>site2.internal</filename>, <filename>site1.example.com</filename>,
        and <filename>site2.example.com</filename>, to the servers
        in the
        DMZ. These internal servers will have complete sets of information
        for <filename>site1.example.com</filename>, <filename>site2.example.com</filename>,<emphasis/> <filename>site1.internal</filename>,
        and <filename>site2.internal</filename>.
      </para>
      <para>
        To protect the <filename>site1.internal</filename> and <filename>site2.internal</filename> domains,
        the internal name servers must be configured to disallow all queries
        to these domains from any external hosts, including the bastion
        hosts.
      </para>
      <para>
        The external servers, which are on the bastion hosts, will
        be configured to serve the "public" version of the <filename>site1</filename> and <filename>site2.example.com</filename> zones.
        This could include things such as the host records for public servers
        (<filename>www.example.com</filename> and <filename>ftp.example.com</filename>),
        and mail exchange (MX)  records (<filename>a.mx.example.com</filename> and <filename>b.mx.example.com</filename>).
      </para>
      <para>
        In addition, the public <filename>site1</filename> and <filename>site2.example.com</filename> zones
        should have special MX records that contain wildcard (`*') records
        pointing to the bastion hosts. This is needed because external mail
        servers do not have any other way of looking up how to deliver mail
        to those internal hosts. With the wildcard records, the mail will
        be delivered to the bastion host, which can then forward it on to
        internal hosts.
      </para>
      <para>
        Here's an example of a wildcard MX record:
      </para>
      <programlisting>*   IN MX 10 external1.example.com.</programlisting>
      <para>
        Now that they accept mail on behalf of anything in the internal
        network, the bastion hosts will need to know how to deliver mail
        to internal hosts. In order for this to work properly, the resolvers
        on
        the bastion hosts will need to be configured to point to the internal
        name servers for DNS resolution.
      </para>
      <para>
        Queries for internal hostnames will be answered by the internal
        servers, and queries for external hostnames will be forwarded back
        out to the DNS servers on the bastion hosts.
      </para>
      <para>
        In order for all this to work properly, internal clients will
        need to be configured to query <emphasis>only</emphasis> the internal
        name servers for DNS queries. This could also be enforced via
        selective
        filtering on the network.
      </para>
      <para>
        If everything has been set properly, <emphasis>Example, Inc.</emphasis>'s
        internal clients will now be able to:
      </para>
      <itemizedlist>
        <listitem>
          <simpara>
            Look up any hostnames in the <literal>site1</literal>
            and
            <literal>site2.example.com</literal> zones.
          </simpara>
        </listitem>
        <listitem>
          <simpara>
            Look up any hostnames in the <literal>site1.internal</literal> and
            <literal>site2.internal</literal> domains.
          </simpara>
        </listitem>
        <listitem>
          <simpara>Look up any hostnames on the Internet.</simpara>
        </listitem>
        <listitem>
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          <simpara>Exchange mail with both internal and external people.</simpara>
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        </listitem>
      </itemizedlist>
      <para>
        Hosts on the Internet will be able to:
      </para>
      <itemizedlist>
        <listitem>
          <simpara>
            Look up any hostnames in the <literal>site1</literal>
            and
            <literal>site2.example.com</literal> zones.
          </simpara>
        </listitem>
        <listitem>
          <simpara>
            Exchange mail with anyone in the <literal>site1</literal> and
            <literal>site2.example.com</literal> zones.
          </simpara>
        </listitem>
      </itemizedlist>

      <para>
        Here is an example configuration for the setup we just
        described above. Note that this is only configuration information;
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        for information on how to configure your zone files, see <xref linkend="sample_configuration"/>.
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      </para>

      <para>
        Internal DNS server config:
      </para>

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<programlisting>
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acl internals { 172.16.72.0/24; 192.168.1.0/24; };

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acl externals { <varname>bastion-ips-go-here</varname>; };
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options {
    ...
    ...
    forward only;
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    forwarders {                                // forward to external servers
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        <varname>bastion-ips-go-here</varname>;
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    };
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    allow-transfer { none; };                   // sample allow-transfer (no one)
    allow-query { internals; externals; };      // restrict query access
    allow-recursion { internals; };             // restrict recursion
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    ...
    ...
};
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zone "site1.example.com" {                      // sample master zone
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  type master;
  file "m/site1.example.com";
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  forwarders { };                               // do normal iterative
                                                // resolution (do not forward)
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  allow-query { internals; externals; };
  allow-transfer { internals; };
};